A method and apparatus for analyzing light are provided. Light having spectral components periodic in frequency is collected, collimated and transmitted by light conditioning means to a first interferometric means. The first interferometric means selectively separates a preselected periodic spectrum from the light and transmits such spectrum in the form of a fringe having an intensity derived from and substantially equal to the combined intensities of the transmitted spectra. A second interferometric means adapted to receive the fringe transmits the preselected periodic spectrum thereof in the form of a modified fringe providing a detectable signal. The portion of total intensity of the modified fringe derived from the combined intensities of the transmitted spectrum is greater than that of the fringe in the presence of unwanted spectra.